Reasons:
The grounds for classification of invalidity cases can be grouped under three main headings: initial impossibility, illegality, and immorality.
A- Initial Impossibility: This refers to the impossibility of the subject matter of the legal transaction at the time the legal transaction is established. In this case, the legal transaction is invalid. For example, a contract for the sale of a completely burned-down house renders this legal transaction invalid. In other words, something that does not exist cannot be sold. For initial impossibility to be asserted and transferred, the situation must be objective. In Rome, the sale of items used for religious purposes was not permitted. Therefore, a contract for the sale of religious artifacts is invalid.
B- Illegal:
Legal transactions are invalid if they are contrary to the law. Although the sources of law changed according to the political periods of Rome, the sources of law were, in particular, the decisions of the people’s assembly, the decisions of the praetor, the orders of the emperor, and the Corpus Iuris Civilis during the Justinian period. Therefore, if a legal transaction was performed contrary to these legal sources, the legal transaction was invalid. According to Roman property law, transactions were performed according to the distinction between “res mancipi” and “res nec mancipi.”
For these, the legal rituals of ‘mancipatio’ and “in iure cessio” were performed. These procedures were prescribed by law. Therefore, the transfer of ownership of goods had to be carried out according to these procedures. If the transfer of ownership of goods was not carried out according to these procedures, the legal transactions would be invalid. Again in Rome, there was an oral legal transaction called “stipulatio,” which was frequently seen in debt law. This transaction was a legal transaction established by specific words mutually agreed upon. Here, the words that the parties had to say to each other formed the shape of the transaction. Therefore, legal transactions that did not comply with the formal requirements of the contract were considered invalid.
C- Immorality: The subject and purpose of legal transactions in Rome must not be contrary to morality. Therefore, legal transactions made contrary to general moral rules are invalid. For example, the contract of a company established for the purpose of illegal business is void.